The justices appeared to be divided along the court's ideological fault line, with four conservative justices favoring Arizona and four liberals favoring the state law's challengers, leaving Justice Anthony as a possible swing vote.
SCOTUSBLOG.com reported Kennedy focused on whether an appeals court ruling striking down the state law used the correct constitutional analysis. A Huffington Post report also focused on Kennedy, who appeared to be arguing for both sides of the case from the bench.
Justice Antonin Scalia led conservatives in support of the state law.
A decision in the case could shape the national political landscape for some time. Seventeen states have enacted laws requiring the presentation of some type of government-issued photo identification, such as a driver's license, before voting. The Brennan Center for Justice said those 17 states account for 218 of the 270 electoral votes needed to win the presidency.
The case, Arizona vs. The Inter Tribal Council of Arizona Inc. et al., first reached the U.S. Supreme Court in 2006's Purcell vs. Gonzalez. Though the justices did not reach a resolution, they sent the case back down to the lower courts.
A federal judge refused to issue an injunction against the implementation of the law in the case brought by a coalition of Hispanic, Indian and civil rights groups. However, a federal appellate panel, and then the entire 9th U.S. Circuit Court of Appeals in San Francisco reversed, saying the law violated Article I of the U.S. Constitution and the National Voter Registration Act, which requires states to make it easier to register for federal elections.
Article I says. in part, the "times, places and manner of holding elections for senators and representatives, shall be prescribed in each state by the legislature thereof; but the Congress may at any time by law make or alter such regulations, except as to the places of choosing senators."
Arizona then successfully asked the U.S. Supreme Court for review.